title: Surface water quality is improving due to declining atmospheric N deposition.
abstract: We evaluated long-term surface water nitrate and atmospheric nitrogen (N) deposition trends for a group of nine predominantly forested Appalachian Mountain watersheds during a recent multidecadal period (1986-2009) in which regional NOx emissions have been progressively reduced. Statistical analysis showed unexpected linear declines in both annual surface water nitrate-N concentrations (mean =46.4%) and yields (mean =47.7%) among the watersheds corresponding to comparable declines in annual wet N deposition (mean =34.4%) resulting from U.S. NOx emission control programs during the same time period. Nitrate-N concentration trends were robust across a large geographical region and appeared insensitive to watershed size across several orders of magnitude-suggesting that the improvements in water quality are probably propagated to surface and estuarine waters downstream. Surface waters are thus responding to declining atmospheric N deposition in much the same way they responded to declining sulfur deposition-although only one watershed showed a 1:1 relationship. Application of a kinetic N saturation model indicated that all nine forested watersheds are exhibiting signs of N saturation as evidenced by a limited, but variable, efficiency of demand for N. Further reductions in N deposition would be expected to produce additional reductions in streamwater N loads.
authors.affiliation: University of Maryland Center for Environmental Science , Appalachian Laboratory, Frostburg, Maryland 21532, United States.
authors.forename: Keith N
authors.initials: KN
authors.lastname: Eshleman
authors.forename: Robert D
authors.initials: RD
authors.lastname: Sabo
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: Kline
mesh_terms: D001061:Appalachian Region; D001272:Atmosphere / Q000737:chemistry*; D005843:Geography; D007700:Kinetics; D008962:Models, Theoretical; D009566:Nitrates / Q000032:analysis; D009584:Nitrogen / Q000032:analysis*; D013997:Time Factors; D060753:Water Quality
chemical_list: D009566:Nitrates; D009584:Nitrogen